The present invention relates to an optical device in which a lens element such as a computer-generated hologram is optically coupled to another optical element. More particularly, it relates to a device in which the alignment of the lens element and the other optical element is simplified.
Known alignment methods include passive methods such as the one disclosed by Tanaka et al. in IEICE Transactions on Electronics, Vol. E80-C, No. 1 (January 1997), pp. 107-111. This method aligns an optical fiber with a laser diode chip by mounting both of them on a supporting substrate referred to as a silicon platform. Photolithographic techniques are used to form a V-groove and fiducial marks simultaneously on the silicon platform. The optical fiber is mounted in the V-groove, the depth of which controls vertical alignment. The laser diode chip is mounted on a solder pad on the silicon platform and positioned in relation to the fiducial marks, which control horizontal alignment. Because of the high accuracy of photolithography, the laser diode chip can be positioned with sufficient precision to couple the emitted laser beam, which has a typical diameter of one to six micrometers (1-6 μm), into the optical fiber within an alignment tolerance of one to two micrometers (1-2 μm).
It would be desirable to extend this passive alignment technique to optical devices that also include lens elements such as a computer-generated holograms (CGH elements), which must be formed on a separate optical substrate. CGH elements are extremely useful, because they can be finely and accurately patterned by the same techniques of photolithography and etching as used to fabricate semiconductor electronic circuits, and because by suitable combinations of etching masks, they can be given not only lens functions such as focusing and collimating, but also prism functions such as deflection.
Accurate alignment of a CGH element formed in an optical substrate with an optical element mounted on a silicon substrate requires precise relative positioning of the two substrates. The required precision is conventionally achieved by active alignment techniques, such as by moving the optical substrate in relation to the silicon substrate while transmitting test light from the optical element, and measuring the amount of light that is coupled through the CGH element. Active alignment, however, has the disadvantages of taking time and requiring expensive test equipment.